(1) Field of the Invention
The present invention relates to a liquid detection sensor, a liquid detection apparatus, and a liquid detection system using the sensor and the apparatus.
(2) Description of the Related Art
Along with recent aging society, various care supplies such as paper diapers and apparatuses detecting incontinence have been developed. As one of the apparatuses, for example, the Japanese Laid Open Patent Nos. 2001-289775 and 2001-325865 disclose a liquid detection sensor using a noncontact tag method and a liquid detection apparatus that detects a state of the liquid detection sensor without contacting the sensor.
The liquid detection sensor disclosed in the above documents comprises a coil L, a capacitor C and a switch SW, and has a configuration example in which these components are connected in series in a loop shape (hereinafter this configuration example is referred to as a “first conventional configuration example”), and another configuration example in which the coil L and the capacitor C are connected in a loop shape, and the switch SW is connected to the capacitor C in parallel (hereinafter this configuration example is referred to as a “second conventional configuration example”).
The switch SW of the liquid detection sensor shown in the first and the second conventional configuration examples includes a metal board having electrical conductivity and flexibility like a flat spring, a metal board having electrical conductivity, an insulator located between ends of the metal boards, a material such as paper or fiber, of which volume is reduced when absorbing water, and so on. The switch SW is structured to maintain both electrodes in a normally open state at an initial stage (when dry), and switches both electrodes into a conducting state when absorbing liquid (moisture such as water).
Therefore, in the liquid detection sensor according to the first conventional configuration example, a resonant circuit is not formed by the coil L and the capacitor C because the switch SW cuts out the conduction at its initial stage. The resonant circuit is formed only when the material, which reduces its volume with water, absorbs water, and the switch SW switches into the conducting state. Contrary to the first conventional configuration example, a resonant circuit in the liquid detection sensor according to the second conventional configuration example is formed by the coil L and the capacitor C when the switch SW cuts out the conduction at the initial stage, and the resonant circuit disappears when the material, which reduces its volume with water, absorbs water and the switch SW switches into the conducting state.
In the initial state of the first conventional configuration example, when a detection radio wave at a resonant frequency is sent from the liquid detection apparatus, a receiving level of the radio wave coming back from the liquid detection sensor does not fall since the resonant circuit is not yet formed. But, when the material that reduces its volume absorbs water and the switch SW switches into the conducting state, the receiving level of the radio level coming back from the liquid detection sensor drops. On the other hand, as the resonant circuit is formed in the initial state of the second conventional configuration example, the receiving level of the radio wave coming back from the liquid detection sensor falls. And, when the material that reduces its volume absorbs water and the switch SW switches into the conducting state, the resonant circuit disappears. Consequently, the receiving level of the radio wave coming back from the liquid detection sensor goes up.
Therefore, even if either of the liquid detection sensors in the first and the second conventional examples is used, it is easy for the liquid detection apparatus to detect a condition of the liquid detection sensor by monitoring two different values in the receiving level of the radio wave at the resonant frequency.
However, because the conventional liquid detection sensors are structured to have a switch SW having a complex configuration, it becomes costly as it gets bigger. Also, paper, fiber or the like is used for the material that reduces its volume when absorbing water in the switch SW. Therefore, once the material absorbs water, it does not restore its original volume as it looses elasticity. Therefore, even when the material that reduces its volume dries out, both flat springs are conductive. As a result, the expensive liquid detection sensor cannot be reused.
Moreover, in the conventional liquid detection apparatus, a detection radio wave at a specific resonant frequency is emitted, and two different values in the receiving level of the radio wave at the resonant frequency are monitored. Therefore, when the resonant frequency of the liquid detection sensor scatters and deviates from the specific resonant frequency of the radio wave, which is emitted from the liquid detection apparatus, the level of the radio wave coming back from the liquid detection sensor does not change in the resonant frequency of the detection electric field emitted from the liquid detection apparatus. Consequently, because a subtle level change cannot be detected with the conventional method that compares two values in the liquid detection apparatus, it is not possible to accurately detect the condition of the liquid detection sensor.